Biology
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p43c1446c&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P43C-1446
Biology
[0424] Biogeosciences / Biosignatures And Proxies, [0714] Cryosphere / Clathrate, [1041] Geochemistry / Stable Isotope Geochemistry, [5215] Planetary Sciences: Astrobiology / Origin Of Life
Scientific paper
Recent reports of persistent trace-level methane in the Martian atmosphere and sulfur-bearing compounds (including sulfate and hydrogen sulfide) over the Jovian satellite Europa suggest dissociation of gas-containing water ice, known as gas hydrates or clathrates containing CH4 and/or H2S. Icy reservoirs containing volatiles are anticipated to be stable in the shallow subsurface of Mars and near the interface of the liquid ocean and solid ice shell on Europa. Of astrobiological interest, the gases trapped within clathrates could be biological in origin and evidence of chemolithotrophic metabolic pathways such as microbial methanogenesis (MOG) and anaerobic oxidation of methane (AOM) coupled with bacterial sulphate reduction (BSR) such as occurs on Earth. If the gases trapped as guest molecules inside clathrates are the product of MOG, AOM, and BSR then it may be possible to infer their origin and differentiate between biological and abiotically generated gases produced through analyzes of C, H, and S stable isotopes. Samples of gas hydrates obtained by drilling in cryosphere environments provide relatively stable solid materials suitable for preservation of biosignatures in both terrestrial and extraterrestrial systems. Notably, diagnosis of biological or geological origin using isotopic measurements is dependent on a full understanding of isotopic fractionation during clathrate formation or dissociation. We endeavor to assess isotopic fractionation during the process of trapping gas molecules at different temperatures and pressures of clathrate formation. Three clathrate nucleation experiments have been conducted: two using methane (249 K and 1.72 MPa; 240 K and 1.21 MPa ) and one using hydrogen sulfide (233 K and 0.2 MPa) as the guest molecules. Carbon isotopic analyses of the recovered gas fractions (reservoir tank, headspace, and clathrate trapped) from the methane experiments show only small differences in δ13C values (Δ13Creservior-headspace, Max=0.19, Min=0.03 ‰) between starting tank gas and remaining headspace gas or gas trapped in clathrate (Δ13C headspace-clathrate, Max=0.63, Min=0.03 ‰). These results are in agreement with previously reported findings and indicate that stables isotopes of carbon in methane are unlikely to be overprinted in ways that would obscure biosignatures if the gases originated from biological processes. Our ongoing laboratory experiments aim to decipher the influence of the clathrate nucleation and dissociation on the isotopic composition of H in methane and S in hydrogen sulfide.
Carvajal-Ortiz H.
Pratt Lisa
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